1. Field of the Invention
The present invention broadly relates to the superplastic forming and diffusion bonding process, and, more particularly, is concerned with a tool sealing arrangement and method which uses the workpiece itself to form the tool seal.
2. Description of the Prior Art
Superplastic forming and concurrent diffusion bonding (hereinafter denoted SPF/DB) is a relatively new technology used to produce sheet metal assemblies or components in a more cost effective manner than by the conventional riveting or welding methods. Only a few metals and alloys, such as titanium and its alloys, are suitable for use with this technology.
One significant part of the SPF/DB process is provision of an adequate tool seal. Various sealing arrangements are disclosed and illustrated in patents to Hamiltion et al, U.S. Pat. No. 3,920,175; Hamilton et al, U.S. Pat. No. 3,927,817; and Hamilton et al, U.S. Pat. No. 3,934,441. Each of these patents uses either compression of the edge of the sheet metal between opposing surfaces of upper and lower tools or structural features on the tool surfaces. In all cases, the seal has to prevent the outside atmospheric gases from entering into the inner area between the tools. This inner area can only accept a benign gas, such as argon, to preclude surface contamination of the titanium sheet metal.
One approach to tool sealing, such as used in the tooling of the aforementioned patents, is to seal upper and lower tools along a common parting plane defined between the tools. However, this seal is a critical cost factor and probably the most difficult technical detail of the tooling system to devise. Generally, the upper and lower tools are massive steel or ceramic constructions which are exposed to the processing conditions of high pressure, such as up to 300 psi, and high temperature, such as 1650.degree. to 1700.degree. F. The temperature gradients through the tooling material during heat-up and cool-down cause the tools to undergo deformations which result in warpage and, therefore, mismatch in the parting planes.
Another approach to tool sealing is to enclose the entire tooling system in a welded sheet metal envelope. However, the costs associated with this arrangement prevent the technology from becomming a cost competitive alternative to conventional fabrication methods. Installing, welding, and removing the envelope requires many labor hours.
Therefore, for the SPF/DB process to become a viable alternative to conventional techniques of welding and riveting for use in fabricating structures, such as wing skin panels for aircraft, from titanium sheet metal, a need exists for a simple, reliable, low cost sealing arrangement about the tooling periphery.